Michael Adams-Conroy died at the age of nine of the highest overdose of Prozac® on record, seven times higher than any previously known. His parents were investigated for homicide and his two siblings were removed from their custody by social welfare workers. Autopsy results, however, showed no pills in his stomach even though he would normally have had to ingest a huge number in order to reach the levels of drug found in his blood.
Acute lymphocytic leukemia (ALL) affects thousands of children each year in the United States. Treatment with chemotherapeutic agents now leads to remission in over 90% of the cases. 6-mercaptopurine (6-MP) is one agent used to treat ALL. However, the normal treatment dose of 6-MP is toxic for one in 300 patients and can kill rather than cure.
Adverse reactions to therapeutic drugs have been estimated to kill over 100,000 hospitalized patients in the U.S. each year (Lazarou et al., JAMA 1998 Apr. 15;279(15):1200–5). This figure does not include intentional overdoses leading to hospitalization which ultimately prove fatal. An additional 2.2 million serious nonfatal adverse drug reactions have been estimated to occur.
The problem of the varied responses of individual patients to particular drug therapies is well known, but little progress has been made towards anticipating patients' varied drug metabolisms prior to treatment. The standard approach in administering drugs has been to prescribe the recommended dosage for a given condition to an affected patient, in some cases adjusting for the patient's weight. If the patient does not improve, the dosage is increased or an alternative drug is tried. Conversely, if adverse side effects occur, the dosage may be lowered or an alternative drug employed.
Drugs which exhibit serious side effects may never be approved by regulatory authorities or, if approved before such side effects are identified, can be withdrawn from the market if even a small percentage of treated patients are so affected. This can occur despite the fact that such drugs may have great therapeutic benefit in the majority of patients.
The 6-MP sensitivity exhibited by rare ALL patients has been linked to a deficiency in thiopurine S-methyltransferase (TPMT) activity (Krynetski et al., Pharm Res 1999 16(3):342–9). Patients deficient in this enzyme can be treated with lower doses of 6-MP to achieve the same therapeutic plasma levels while avoiding adverse toxicity if the prescribing physician is aware of the metabolic deficiency. Metabolism of similar drugs such as azathioprine and thioguanine used in the treatment of rheumatoid arthritis, leukemia and Crohn's disease is also affected in patients who are deficient in TPMT.
Cytochrome p-450 CYP2D6 (debrisoquin hydroxylase) is the primary enzyme responsible for human metabolism of fluoxetine (Prozac®), as well as codeine, amphetamines, methadone, and several antidepressants and neuroleptics. At least twenty variants of the CYP2D6 gene are now known to result in poor metabolism of Prozac® and other drugs (Wong et al., Ann Acad Med Singapore 2000 29(3):401–6). Approximately 7–10% of Caucasians are poor metabolizers of Prozac®, and reach higher than expected plasma levels when treated with a standard dosage.
Michael Adams-Conroy was one such patient, but he was never tested to determine whether he harbored any of the CYP2D6 variants resulting in slow metabolism of Prozac. Instead, because of his diminished response to Prozac®, as typically occurs with chronic use, his dosage was gradually increased to maintain control over his symptoms. Side effects associated with Prozac® toxicity such as nausea and dizziness were instead attributed to migraines. Only after Michael's death were his tissues tested and shown to contain CYP2D6 variants which contributed to a toxic accumulation of Prozac® and its metabolites in his blood (Sallee et al., J. Child Adolesc. Psychopharmacol. 2000 Spring; 10(1):27–34).
Potentially fatal adverse drug reactions are now known to be associated with altered metabolism by patients harboring variants in a number of genes, including in the NAT2 gene affecting isoniazid metabolism, in the CYP2C9 gene affecting warfarin metabolism, in the DPD gene affecting 5-fluorouracil metabolism, and in the KCNE2 gene affecting clarithromycin metabolism (Grant et al., Pharmacology 2000 61(3):204–11; Taube et al., Blood 2000 96(5):1816–9; Meinsma et al., DNA Cell Bio 1995 14(1):1–6; Sesti et al., Proc Natl Acad Sci USA 2000 97(19):10613–8).
There is a need in the art for methods of analyzing samples for particular polynucleotides, and for devices, compositions and articles of manufacture useful in such methods.